1. Field
Exemplary embodiments of the present invention relate to a technology for fabricating a semiconductor device, and more particularly, to an image sensor including a lightguide and a method for fabricating the same.
2. Description of the Related Art
An image sensor is a device for converting an optical image into an electrical signal. As design rules of devices shrinks, the unit pixel size of image sensors also decreases and causes a decrease in sensitivity of image sensors. Here, such a decrease in sensitivity may increase a signal-to-noise ratio (SNR) and thus degrade the quality of an image acquired through image sensors.
To address such features, a front-side-illumination (FSI) image sensor may use a lightguide. Hereafter, referring to FIG. 1, an image sensor including a lightguide will be described.
FIG. 1 is a cross-sectional view of a conventional image sensor.
Referring to FIG. 1, the conventional image sensor includes a substrate 11 having a plurality of unit pixel regions and a plurality of light receiving elements 13 formed in respective unit pixel regions. The respective light receiving elements 13 are isolated by an isolation layer 12. An interlayer dielectric layer 14 including multilayer metal interconnections 16 is formed on the substrate 11. The interlayer dielectric layer 14 includes a lightguide 15 formed for respective light emitting elements 13. A color filter 17 having an RGB structure is formed on the interlayer dielectric layer 14 so as to cover the unit pixel regions, and a planarization layer 18 is formed on the color filter 17. On the planarization layer 18, a hemispheric lens 19 is formed for respective light receiving elements 13.
In the conventional image sensor, the opening area of the lightguide 15 is limited by the metal interconnections 16 formed in the interlayer dielectric layer 14. Therefore, the conventional image sensor has physical limits in focusing light.
Furthermore, in order to secure an incident angle which satisfies a total reflection condition of light incident on the lightguide 15, adjacent hemispheric lenses 19 are formed to be spaced apart from each other. However, since there are physical limits in securing an incident angle to satisfy the total reflection condition, it is difficult to obtain appropriate margins for the lens 19. Furthermore, light incident between adjacent lens 19 may not reach the light receiving element 13 and be lost.
Meanwhile, when uniform properties are not secured at the edges of each unit pixel region even when adjacent lenses 19 are formed to contact each other, there are physical limits in focusing light incident obliquely with respect to the lens 19 to a target unit pixel region. Thus, optical shading effects may be distorted.
In order to obtain lens 19 which satisfies the light focusing ability and the total reflection condition despite the limited opening area of the lightguide 15, the lightguide 15 is formed of a high refractive index material. In this case, production costs of image sensors may increase.